Many simple and convenient assay device s are now available which comprise a strip or sheet of porous material through which sample liquid progresses and carries a labelled specific binding re agent into a test zone in which the assay result is revealed. Such devices are loosely referred to as "immunochromatographic" devices, although the specific binding reagent t may not necessarily be immunochemical in the strictest sense. Examples of such devices are described in EP 186799, EP 291194 and EP 383619.
In a typical device the sample liquid is applied to one end of a defined liquid flow path. Usually the sample liquid is received by a physically distinct body of porous material, and during the course of the assay must flow from this first body into one or more other porous bodies arranged in series. Usually the successive porous bodies have different characteristics, such as porosity or composition. Usually, in order for the assay to work efficiently, there should be unimpeded flow of sample liquid from one body into the next. Typically the porous bodies are contained, wholly or partially, within a protective casing constructed for example from plastics material. Inevitably there is at least partial contact between one or more inner surfaces of the casing and one or more of the porous bodies. The series of porous bodies can be firmly linked to one another by being fixed to a common support, for example by being laminated onto a single backing strip. Alternatively, adjoining bodies can be held in appropriate contact merely by being constrained within the casing.
Usually the sample liquid flows through the device by capillary action. The intention is that the porous nature of the strip components ensures that the liquid flows only through these components and not elsewhere within the device. However, in a typical device, especially one which has a protective casing, there may be opportunities for the liquid to find alternative flow paths created, for example, by close proximity between an inner surface of the casing and the exterior of one of the porous components. This tendency to deviate from an intended flow path may be exacerbated if a user of the device applies too great a volume of sample to the device. Under such circumstances the device may tend to "flood". Such flooding may cause a significant proportion of the sample liquid within the device to bypass a critical stage in the intended flow path. For example, some of the liquid may reach a test zone without encountering one or more reagents, such as a mobilisable labelled reagent, that have been placed deliberately in the intended flow path. Flooding may therefore lead to a false assay result, or at least lead to a loss of assay sensitivity.
An objective of the present invention is to provide assay devices in which the likelihood of sample liquid deviating from an intended flow path is reduced.